1. Field of the Invention
The present invention relates to a levelling machine for levelling or flattening sheet metal or strip. The levelling machine includes upper and lower levelling rolls which are arranged offset relative to each other and which are supported over their length by means of back-up rolls which, in turn, are supported and arranged in roll stands.
2. Description of the Related Art
A levelling machine of this type has become known from DE 42 00 922 A1. The levelling machine includes upper and lower crossbeams for supporting the back-up rolls. For positioning the levelling rolls, the upper crossbeam is adjustable by means of adjusting cylinders relative to the lower crossbeam. In addition, the levelling rolls of this levelling machine are individually adjustable and individually driven and, for this purpose, are connected through universal joint shafts to a drive. Bearing support wedges and displaceable adjusting wedges which are each displaceable by means of, for example, a pressure medium cylinder, are provided in the lower and upper roll stand for adjusting individual levelling rolls, i.e., for raising or lowering the levelling rolls. Accordingly, the hydraulically operated wedge pairs used as adjusting members for the levelling rolls including the back-up rolls make it possible to remove certain upper and lower rolls from the levelling process and, thus, to adjust from levelling roll to levelling roll different and possibly asymmetrical graduations or spacings and, thus, varying lever conditions. As a result, smaller forces occur at the levelling rolls arranged with a greater degree of graduation as compared to the normal adjustment. These smaller forces make it possible to level thicker sheet metal or strip, wherein the thickness can be increased until the full force again occurs at each levelling roll.
It has been found that problems occur in levelling machines whose levelling rolls are driven individually when it is necessary to level high-strength and additionally thin sheet metal or strip, for example, with a yield point of 1700 N/mm.sup.2 as it is usual in spring steel and with a thickness of approximately 2 mm. In that case, the levelling rolls must have a diameter which is substantially smaller than in conventional plants, for example, 45 mm, wherein, in addition, the required high torques must be introduced. Since the levelling force at the levelling rolls is smaller in the case of thinner sheet metal, there are difficulties with respect to the transport of the sheet metal ensured by the frictional contact between the levelling rolls and the sheet metal or strip.